1. Field of the Invention
The present invention relates to a driving device typically implemented as an improved hollow cylindrical stepper motor.
2. Description of the Related Art
Conventionally, there has been proposed a stepper motor which is reduced in the diameter about a rotor shaft (rotary shaft) thereof and is at the same time enhanced in output power (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H09-331666). Now, a stepping motor (stepper motor) according to the prior art disclosed in this publication will be described with reference to FIGS. 13 and 14.
FIG. 13 is an exploded perspective view of the conventional stepper motor, and FIG. 14 is a cross-sectional view of the stepper motor shown in FIG. 13, in a state in which the assembly of the stepper motor is completed.
As shown in FIGS. 13 and 14 the stepper motor is comprised of a rotor 201, a first coil 202, a second coil 203, a first stator 204, a second stator 205, an output shaft 206, and a connection ring 207.
The rotor 201 has a hollow cylindrical shape, and is formed by a permanent magnet (magnet) which is circumferentially divided into four sections which are magnetized such that they have alternately different poles. The first coil 202 and the second coil 203 are arranged adjacent to the rotor 201 on axially opposite sides of the rotor 201. The first stator 204 is formed of a soft magnetic material, and is magnetized by the first coil 202. The second stator 205 is formed of a soft magnetic material, and is magnetized by the second coil 203.
The first stator 204 includes first outer magnetic pole parts 204A and 204B which are opposed to an outer peripheral surface of the rotor 201 with a gap between the first outer magnetic pole parts 204A and 204B and the outer peripheral surface of the rotor 201, and first inner magnetic pole parts 204C and 204D which are opposed to an inner peripheral surface of the rotor 201 with a gap between the first inner magnetic pole parts 204C and 204D and the inner peripheral surface of the rotor 201. The second stator 205 includes second outer magnetic pole parts 205A and 205B which are opposed to the outer peripheral surface of the rotor 201 with a gap between the second outer magnetic pole parts 205A and 205B and the outer peripheral surface of the rotor 201, and second inner magnetic pole parts 205C and 205D which are opposed to the inner peripheral surface of the rotor 201 with a gap between the second inner magnetic pole parts 205C and 205D and the inner peripheral surface of the rotor 201.
The output shaft 206 has the rotor 201 rigidly secured thereto, and is rotatably held by a bearing 204E of the first stator 204 and a bearing 205E of the second stator 205. The connection ring 207 is formed of a non-magnetic material, and holds the first stator 204 and the second stator 205 with a predetermined gap between the first stator 204 and the second stator 205.
With the construction described above, the energizing direction of the first coil 202 and that of the second coil 203 are switched to thereby switch the polarities of the first outer magnetic pole parts 204A and 204B, the first inner magnetic pole portions 204C and 204D, the second outer magnetic pole parts 205A and 205B, and the second inner magnetic pole parts 205C and 205D. This causes the rotor 201 to keep rotating.
In the above-described stepper motor, magnetic fluxes generated by energization of the first coil 202 and the second coil 203 are allowed to flow from the outer magnetic pole parts to the inner magnetic pole parts radially opposed thereto, or alternatively from the inner magnetic pole parts to the outer magnetic pole parts radially opposed thereto, so that the magnetic fluxes efficiently act on the magnet forming the rotor 201 located between the outer magnetic pole parts and the respective associated inner magnetic pole parts. Further, the distance between each outer magnetic pole part and the associated inner magnetic pole part can be set to a value almost equal to the thickness of the hollow cylindrical magnet, and hence it is possible to reduce the resistance of a magnetic circuit formed by the outer magnetic pole parts and the inner magnetic pole parts. As the resistance of the magnetic circuit is smaller, a larger amount of magnetic flux can be generated with a smaller amount of electric current, which leads to the enhancement of output power of the stepper motor.
Further, a stepper motor as a further improvement of the above-described stepper motor has been proposed (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H10-229670). In the proposed stepper motor, inner magnetic pole parts are formed as parts of hollow cylindrical members, with an output shaft made of a soft magnetic material being inserted into the respective holes of the hollow cylindrical members, and bearings made of a non-magnetic material are attached to a stator provided with the inner magnetic pole parts and outer magnetic pole parts, for rotatably holding the output shaft. According to the proposed stepper motor, the output shaft as well can be used as a component of the magnetic circuit, which contributes to an increase in the output power of the stepper motor increased.
However, the stepper motors disclosed in Japanese Laid-Open Patent Publications (Kokai) No. H09-331666, and No. H10-229670 both necessitate provision of predetermined gaps between the inner peripheral surface of the magnet forming the rotor and the outer peripheral surfaces of the inner magnetic pole parts opposed thereto. The control of the predetermined gaps during manufacturing of the stepper motors results increase manufacturing costs. Further, although the stators are required to be formed with hollow cylindrical inner magnetic pole parts and outer magnetic pole parts, it is difficult to integrally form the inner magnetic pole parts and the outer magnetic pole parts. Further, in the case where the inner magnetic pole parts and the outer magnetic pole parts are separately formed, and then assembled together into one piece, this increases the number of component parts, which results in an increase in the manufacturing costs.
Further, in the case of the stepper motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H09-331666, if the first outer magnetic pole parts of the first stator and the second outer magnetic pole parts of the second stator are made closer to each other, crosstalk is induced therebetween, whereby rotational accuracy and rotational output are degraded. To solve this problem, the gap T1 is axially provided between the first outer magnetic pole parts and the second outer magnetic pole parts.
Assuming that the number of the magnetic poles of the magnet forming the rotor is equal to n, the first outer magnetic pole parts and the second outer magnetic pole parts are arranged on the outer peripheral surface of the rotor in a manner shifted in phase by (180/n) degrees. Moreover, the first outer magnetic pole parts are arranged at n/2 locations at an angular pitch of 720/n degrees through an opposed angle of not more than 360/n degrees with respect to the outer peripheral surface of the rotor, and the second outer magnetic pole parts are also arranged at n/2 locations at an angular pitch of 720/n degrees through an opposed angle of not more than 360/n degrees with respect to the outer peripheral surface of the rotor. Therefore, unless the gap T1 is axially provided between the first outer magnetic pole parts and the second outer magnetic pole parts, the former and the latter are brought into contact with each other.
Further, since the gap T1 is provided between the first outer magnetic pole parts and the respective associated second outer magnetic pole parts, the axial length of the first outer magnetic pole parts opposed to the rotor is equal to (ML−T1)/2 where ML represents the axial length of the rotor. This means that the rotor is not effectively utilized, and particularly when the axial size of the stepper motor is reduced, the output power of the stepper motor is lowered.